<_j*~V. 


BULLETIN  No.  187 


THE  INFLUENCE  OF  TREES  AND  CROPS 
ON  INJURY  BY  WHITE-GRUBS 


BY  STEPHEN  A.  FOEBES 
STATE  ENTOMOLOGIST 


URBANA,  ILLINOIS,  FEBRUARY,  1916 


THE  INFLUENCE  OF  TREES  AND  CROPS 
ON  INJURY  BY  WHITE-GRUBS 

BY  STEPHEN  A.  FORBES,  STATE  ENTOMOLOGIST 

White-grubs  most  abundant  in  the  Neighborhood  of  Trees. — The 
fact  that  our  common  May-beetles,  the  parents  of  most  of  our  white- 
grubs,  fly  back  and  forth  each  morning  and  evening  between  the  trees 
upon  which  they  feed  at  night  and  the  ground  on  which  they  hide  by 
day  and  in  which  they  lay  their  eggs,  and  the  further  fact  that  they 
are  never  seen  to  move  long  distances  from  place  to  place,  lead  to  the 
natural  supposition  that  fields  nearest  to  their  food-plants  must  be- 
come most  heavily  stocked  with  eggs,  and  consequently  worst  injured 
by  grubs  when  these  eggs  have  hatched.  It  has,  in  fact,  been  fre- 
quently noticed  that  this  seems  to  be  the  case ;  and  it  was  for  the  pur- 
pose of  getting  definite  information  on  the  subject,  in  a  form  for  state- 
ment in  ratios  of  frequency  or  degrees  of  injury,  that  I  began  in  1904 
to  instruct  my  field  assistants  to  make  collections  of  white-grubs  from 
fields  which  were  being  plowed  either  in  fall  or  spring.  For  this  pur- 
pose they  walked  behind  the  plowman,  making  note  of  the  distance 
which  they  traveled  in  each  field,  counting  the  grubs  exposed  by  the 
plow,  and  recording  at  the  same  time  the  distance  from  the  field  to  the 
nearest  trees  upon  which  the  May-beetles,  the  parents  of  the  white- 
grubs,  might  be  supposed  to  have  fed.  Their  data  concerning  abun- 
dance of  the  grubs  were  recorded  in  the  form  of  numbers  per  mile  of 
furrow  traveled.  The  data  sheets  gave  also  the  locality  and  date  of 
each  collection,  the  name  of  the  owner  of  the  field,  the  crop  of  the 
year  and  of  the  four  years  immediately  preceding,  the  character  and 
classification  of  the  soil,  its  level — whether  upland  or  lowland — its 
condition  as  to  drainage,  and  the  kinds  of  trees  in  the  neighborhood, 
as  well  as  the  distance  of  these  trees  from  the  margins  of  the  field. 
Observations  and  collections  of  this  sort  were  continued  by  six  of  my 
field  assistants  as  opportunity  offered,  during  the  years  1904,  1905, 
1907,  and  1908.  In  this  time  white-grubs  were  collected  from  five 
hundred  and  forty-nine  fields  widely  scattered  thruout  central  Illinois, 
with  a  few  fields  also  in  the  northern  and  southern  parts  of  the  state. 
The  total  distance  traveled  in  this  pursuit  was  a  trifle  over  four  hun- 
dred and  twenty-nine  miles,  and  the  total  number  of  grubs  of  the 
genus  Phyllophaga  obtained  was  12,069.  In  addition  to  these,  1187 
grubs  of  the  genus  Cyclocephala  were  collected,  of  which,  however,  no 

26\ 


262  BULLETIN  No.  187  [February, 

account  can  be  taken  in  this  discussion,  since  the  beetles  of  this  genus 
do  not  feed  on  the  leaves  of  trees. 

One  hundred  miles  of  additional  travel  behind  the  plow  in  1909 — 
77.5  miles  in  Kane  county  and  22.5  miles  in  Marion  and  Perry  coun- 
ties— proved  on  analysis  to  have  been  done  in  localities  where  white- 
grubs  were  so  few  that  the  data  of  that  year  could  not  be  used  in  this 
discussion.  The  northern  Illinois  fields  averaged  only  eight  grubs  to 
the  mile,  instead  of  the  average  of  twenty-eight  to  the  mile  of  the 
earlier  collections,  and  most  of  these  fields  yielded  only  one  or  none  in 
the  same  distance.  The  southern  Illinois  fields,  altho  practically  sur- 
rounded by  woodlands  and  orchards,  gave  us  only  twelve  grubs  to  the 
mile  in  1909,  and  lacked  the  contrast  of  surrounding  conditions  nec- 
essary to  the  inquiry. 

For  purposes  of  classification,  my  data  sheets  were  assorted  into 
four  groups,  according  to  the  distances  of  the  fields  from  the  nearest 
trees — group  1  relating  to  fields  with  trees  within  or  on  their  borders 
or  within  less  than  an  eighth  of  a  mile ;  group  2,  to  those  with  trees 
more  than  an  eighth  but  less  than  a  fourth  of  a  mile  away ;  group  3, 
to  those  with  trees  more  than  a  quarter  of  a  mile  away  and  less  than 
a  half ;  and  group  4,  to  those  with  trees  half  a  mile  away  or  more.  Two 
hundred  and  twenty-four  miles  were  traveled  in  fields  belonging 
under  group  1,  with  the  result  that  white-grubs  of  the  genus  Phylloph- 
aga were  found  at  an  average  rate  of  39.17  to  the  mile ;  forty  miles 
were  traveled  in  fields  of  group  2,  and  in  these  Phyllophaga  grubs  aver- 
aged 17.83  to  the  mile ;  in  fields  of  group  3,  a  hundred  and  thirty  miles 
were  traveled,  and  Phyllophaga  grubs  averaged  15.94  to  the  mile ;  while 
in  group  4,  thirty-five  miles  were  traveled,  giving  14.4  as  the  average 
number  of  Phyllophaga  grubs.  From  this  it  would  appear  that  fields 
so  situated  with  reference  to  fruit,  shade,  or  forest  trees  that  May- 
beetles  might  feed  within  them,  on  their  borders,  or  within  less  than 
an  eighth  of  a  mile  away,  contained  two  and  a  fifth  times  as  many 
white-grubs  as  those  whose  nearest  trees  were  between  an  eighth  and 
a  quarter  of  a  mile  distant ;  that  they  contained  practically  two  and 
a  half  times  as  many  grubs  as  those  whose  nearest  trees  were  between 
a  quarter  and  a  half  mile  away;  and  that  they  contained  two  and 
three  quarters  times  as  many  grubs  as  those  with  trees  half  a  mile 
away  or  more. 

No  cttempt  was  made,  in  collating  these  data,  to  distinguish  be- 
tween the  different  kinds  of  trees — a  useless  task,  as  the  different 
species  of  beetles  have  different  preferences  as  to  food ;  and  it  is  im- 
possible to  separate  the  grubs  by  species.  Ample  materials  on  this 
subject  are  contained  in  a  paper  entitled  "A  General  Survey  of  the 
May-beetles  (Phyllophaga)  of  Illinois"  (Bull.  186  of  this  series). 

The  Kinds  of  Crops  in  which  May-beetles  Prefer  to  lay  their 
Eggs, — The  information  derived  from  these  collections  has  enabled  me 


1916] 


WHITE-GRUB  INJURY  AS  BELATED  TO  TRESS  AND  CROPS 


263 


to  distinguish  also  between  the  different  kinds  of  crops  with  reference 
to  the  preferences  of  May-beetles  in  searching  for  places  to  lay  their 
eggs.  It  has  been  well  established,  mainly  by  the  breeding-cage  work 
of  John  J.  Davis,  of  the  U.  S.  Bureau  of  Entomology,  that  our  most 
abundant  May-beetles  have  a  three-year  life  cycle  in  Illinois.  From 
eggs  laid  by  the  beetles  in  spring  or  summer,  grubs  hatch  the  same 
season,  and  then  pass  thru  practically  two  entire  years  in  the  larval 
or  grub  stage,  and  pupate  in  the  third  summer  or  fall  after  the  eggs 
were  deposited.  From  these  pupae  the  beetles  are  formed  under- 
ground, where  they  pass  the  winter,  coming  out  at  the  end  of  the  third 
full  year  to  lay  their  eggs. 

From  this  outline  it  is  evident  that  white-grubs  fairly  well  grown 
in  fall  are  from  eggs  deposited  the  year  before,  and  that  if  one  knows 
the  crop  on  the  field  that  year  he  may  say  with  confidence  that  the 
parent  beetles  laid  their  eggs  in  that  crop.  Well-grown  grubs  seen 
in  spring  and  early  summer,  on  the  other  hand,  may  have  come  from 
eggs  laid  either  the  preceding  year  or  a  year  still  earlier.  To  be  sure, 
consequently,  of  the  crop  in  which  the  eggs  which  gave  origin  to  these 
spring  grubs  were  laid,  we  must  know  what  crops  the  field  has  borne 
for  the  two  preceding  years,  and  can  make  use  only  of  those  fields  in 
which  the  crops  have  been  the  same  both  years.  If  such  a  field  has 
been  in  corn,  for  example,  for  the  last  two  years  before  the  time  of 
observation,  it  is  certain  that  the  parent  beetles  of  the  spring  white- 
grubs  must  have  laid  their  eggs  in  corn,  whatever  the  age  of  the  grubs. 
By  classifying  my  data  sheets  in  accordance  with  these  facts,  I  deter- 
mined positively  the  crops  in  which  the  eggs  were  laid  that  had  pro- 
duced 9664  of  our  white-grubs,  obtained  during  two  hundred  and 
ninety-one  miles  of  travel  behind  the  plow.  The  following  table  shows, 
for  each  crop  in  which  May-beetles  had  laid  their  eggs,  the  number  of 
miles  which  my  collectors  traveled  in  making  their  collections,  the 
total  number  of  grubs  obtained,  and  the  number  per  mile. 

TABLE  SHOWING  CROPS  IN  WHICH  EGGS  MUST  HAVE  BEEN  LAID  FROM  WHICH  WHITE- 
GRUBS    COLLECTED   WERE    HATCHED,   TOGETHER   WITH    THE    WHOLE   NUMBER 
OF  GRUBS   COLLECTED,  AND  THE  NUMBER  PER  MlLE  FOR  EACH 

SUCH  CROP 


Crop  on  ground  when 
eggs  were  laid 

Miles  traveled 

Number  of  grubs 
collected 

Number  of  grubs 
per  mile 

Corn 

177.1 

4342 

24.5 

Meadow   crops, 

excluding  clover 

37.5 

542 

14.7 

Pasture 

23.5 

1991 

84.3 

Wheat 

20.5 

1272 

62. 

Fallow  ground 

18.5 

812 

43.3 

Oats 

9.38 

571 

00.9 

Clover 

4.5 

134 

29.7 

290.98 

9664 

264  BULLETIN  No.  187  [February, 

From  this  table  we  find  that  more  eggs  were  laid  in  pastures  (84 
grubs  per  mile)  than  in  any  other  crop;  that  small  grain  came  next 
with  61  and  62  per  mile  for  fields  which  had  been  in  oats  and  wheat 
respectively ;  that  fallow  land,  grown  up  of  course  to  weeds,  largely 
grasses,  was  third  with  48  grubs  to  the  mile;  that  clover  and  corn 
seemed  not  far  apart  in  attractiveness  to  the  egg-laying  beetles — clover 
with  30  and  corn  with  25  grubs  to  the  mile;  and  that  meadow  crops 
(excluding  clover)  were  least  sought  by  the  egg-laying  beetles — about 
15  grubs  to  the  mile  in  fields  which  had  been  in  such  crops  when  the 
eggs  were  laid. 

Additional  light  is  thrown  on  this  subject  of  a  possible  choice  of 
crops  by  May-beetles  in  which  to  lay  their  eggs,  by  data  of  quite  an- 
other description  obtained  in  1912.  This  was  a  year  of  extraordinary 
injury  by  white-grubs  in  several  counties  of  northern  Illinois;  and 
a  request  for  information  concerning  the  injury  itself  and  the  pre- 
vious cropping  of  the  infested  fields  was  sent  out  (during  my  absence 
in  Europe)  by  Mr.  Robert  D.  Glasgow,  then  of  my  office  staff.  Sixty- 
three  replies  to  this  inquiry  were  received  from  farmers  in  Carroll, 
Stephenson,  Winnebago,  and  Whiteside  counties.  In  thirty-five  of 
these,  estimates  were  given  of  the  injury  to  fields  of  corn.  The  total 
area  of  these  thirty-five  fields  was  676  acres,  an  average  of  19.3  acres 
to  the  field ;  and  the  total  estimated  injury  was  $88.43,  an  average  of 
$13.08  per  acre.  Forty-four  of  the  sixty-three  fields  reported  had  been 
injured  by  white-grubs.  Thirty-five  of  them  were  in  corn  when  in- 
jured, six  in  grass,  and  three  in  potatoes.  The  injury  was  uniformly 
reported  as  continuing  thruout  the  season  from  May  or  June  to  Sep- 
tember, and  it  was  certain,  consequently,  that  these  grubs  had  hatched 
from  eggs  laid  in  1911.  Of  the  forty-four  injured  fields,  nineteen  were 
in  grass  in  1911,  nineteen  were  in  oats,  three  in  rye,  one  in  barley,  and 
two  in  corn.  That  is,  altho  80  percent  of  the  injured  fields  were  in 
corn  in  1912  only  5  percent  of  them  had  been  in  that  crop  when  the 
parent  beetles  laid  their  eggs ;  and  altho  only  14  percent  of  the  injured 
fields  were  in  grass  in  1912,  43  percent  of  them  had  been  in  that  crop 
the  preceding  year.  None  of  the  injured  fields  was  in  oats,  rye,  or 
barley  in  1912,  but  52  percent  of  them  had  been  in  those  crops  in 
1911.  Or,  more  briefly  stated,  in  95  percent  of  the  injured  fields  the 
eggs  had  been  laid  in  either  small  grains  or  grasses,  altho  only  14 
percent  of  the  injury  was  in  fields  bearing  such  crops. 

Turning  to  the  nineteen  uninjured  fields,  on  the  other  hand,  I  find 
that  fourteen  of  them  were  in  corn  in  1912  and  also  the  same  num- 
ber in  1911,  three  in  grass  in  both  years,  one  in  wheat  in  both  years, 
one  in  rye  in  1912,  and  one  in  oats  in  1911.  Or,  taking  corn,  grass, 
and  the  small  grains  separately,  the  number  of  uninjured  fields  in 
each  of  these  classes  was  the  same  in  both  years.  The  evidence  of  the 
predominance  of  grasses  and  small  grains  over  corn  and  other  crops 


1916]  WHITE-GRUB  INJURY  AS  RELATED  TO  TREES  AND  CROPS  265 

as  a  lure  to  May-beetles  about  to  lay  their  eggs  is  unmistakable  here, 
and  much  more  emphatic  than  that  obtained  from  our  general  col- 
lections behind  the  plow.  This  is,  indeed,  what  we  should  expect,  as 
any  actual  preference  of  one  crop  over  another  would  be  much  more 
plainly  manifest  when  extraordinary  numbers  of  May-beetles  were 
abroad  than  when  we  were  dealing  with  a  sparse  and  widely  scat- 
tered population. 


ERRATUM 

Page  264,  line  22,  for  $88.43  read  $8843. 


UNIVERSITY  OF  ILLINOIS-URBANA 


